chore(nix): add formatter

Author: duesee

flake.nix

@@ -31,6 +31,14 @@
         };
     in
     {
+      formatter = eachSupportedSystem (
+        system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+        in
+        pkgs.nixfmt-tree
+      );
+
       devShells = eachSupportedSystem mkDevShells;
     };
 }

shell.nix

@@ -1,13 +1,14 @@
 # Compatiblity file for non-flake Nix users.
 #
 # <https://github.com/edolstra/flake-compat>
-(import
-  (
-    let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
-    fetchTarball {
-      url = lock.nodes.flake-compat.locked.url or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
-      sha256 = lock.nodes.flake-compat.locked.narHash;
-    }
-  )
-  { src = ./.; }
-).shellNix
+(import (
+  let
+    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  in
+  fetchTarball {
+    url =
+      lock.nodes.flake-compat.locked.url
+        or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  }
+) { src = ./.; }).shellNix